The present invention relates in general to a cold shield, and more particularly, to a cold shield for shielding a cryogenic camera and reducing thermal noise of images registered by the cryogenic camera.
Image registration is a fundamental task required in many applications of image processing which involves taking two or more images and aligning them so as to either eliminate differences between them or highlight the salient differences for the purpose of study. To obtain sub-pixel accuracy of the registration, a well registered and stable camera configuration is required. An integrated lens has been used in the single element multiple field cameras. In the integrated lens design, the lens position is constantly referenced to the same mechanical plane referred as a focal plane for minimizing allowable movements critical to maintaining alignment. However, since the focus and alignment of the camera are sensitive to temperature, there is a demand for applying a cryogenic system to the camera to suppress the noise caused by thermal radiation.
In U.S. Pat. No. 5,479,015, a multi-image detector assembly has been disclosed. The multi-image detector assembly allows image detection occurring continuously and simultaneously so as to provide a spatially and temporally correlated set of separate images utilizing a single focal plane. However, the cryogenic application of such multi-image detector assembly was not addressed in this patent.
The cryogenic camera is typically disposed in a cryo-vacuum dewar. For image registration, the cryogenic camera is cooled down to a cryogenic temperature, such that the thermal noise can be effectively reduced, and a higher signal to noise (S/N) ratio can be obtained. A positioning stage is disposed in the cryo-vacuum dewar for adjusting lens positions of the cryogenic camera. The positioning stage comprises several screws allowing the user to adjust the lens position by turning the screws. Calculations are made as to how many screw turns are required for each movement of the lens. Such calculations are done coarsely without any feedback. Sometimes laser is radiated on and reflected from the lens as a reference for determining how much movement of the lens has been made. However, as the positioning stage is not accessible from outside of the dewar, when a new position of the lens is required, the lens and the focal plane array have to be warmed up to room temperature for adjustment of the lens position relative to the focal plane. The warming up procedure involves with many steps such as removing the vacuum, opening the dewar, and re-adjusting the lens position. It then consumes hours in pressurizing, re-evacuating and re-cooling to obtain the desired precision of image detection. Further, as the lens is supported by the positioning stage in a floating manner relative to the focal plane array, misalignment between the lens and the focal plane array occurs every time when the system is warmed up. It is thus very laborious for obtaining the optimal lens positions relative to the focal plane.
It is there a substantially need to provide a cryogenically cooled camera of which the positions of the lens assembly relative to the focal plane array are maintained within a tolerable range even when the temperature is elevated, such that when the camera is warmed, readjustment with desired accuracy can be easily achieved within a short time.